The present invention relates generally to electronic circuits and, more particularly, to a charge coupled device, which provides optical black and offset correction.
FIG. 1 illustrates, in block diagram form, a charge coupled device (CCD), which is an integrated array of photocells used in digital imaging. The analog front end of a CCD generally consists of three main elements: a correlated double sampler (CDS), a programmable gain amplifier (PGA), and an analog to digital converter (ADC).
For each CDS output, two CCD outputs are sampled (FIG. 2a). The first sample is taken during reference, and the second sample is taken during video signal. The difference is the corresponding CDS output. In this way, low frequency noise of the CCD signal is canceled.
As shown in FIG. 2b, a dark cell does not produce a zero differential output. This is due to the dark currents of the photocells, and the effect may vary from line to line in a frame. This dark current value will be referred as optical black level in this text. Due to the optical black level, and the internal offsets of all amplifiers used in the CDS, PGA and ADC, the resulting ADC output for a dark cell will not be zero. As can be seen from FIG. 1, the CDS offset and the optical black level will also be multiplied with the PGA gain. In order to achieve the ideal dynamic range for the signal (FIG. 2b), the black level and the offsets have to be canceled.
There are shortcomings in the currently known methods for canceling the offsets and black levels. In switched capacitor amplifiers, the conventional way to cancel the offset is accomplished by putting the amplifier in unity gain feedback during sampling phase. This way the input offset is also sampled and canceled during the amplification phase. However, for applications where high closed loop gain and high speed are required, the amplifiers may be optimized for high closed loop gain and may not be stable at unit gain feedback.
In continuous time PGAs, the black level can be canceled by putting the PGA in closed loop feedback during optical black period and feeding back the integrated error to the input of PGA as shown in FIG. 3. However, this scheme needs an accurate reference, and the desired ADC optical black level output will not be programmable by the user. In addition, if the PGA is not continuous time but discrete time (switched capacitor), because of the latency of the PGA continuous time feedback will not be possible.
The circuit arrangement shown in FIG. 4 works with discrete time PGAs, but it will take an unknown number of repetitions (lines) to cancel the offsets and optical black level. Also if the PGA gain is too high, the accuracy of the cancellation will be very poor.
In accordance with the present invention, the sum of the channel offset and optical black level is averaged for a given number of lines and optical black cells per line, and the channel is digitally calibrated to obtain a user programmed ADC output which corresponds to that average.